drm/i915/dp_mst: Enable link training fallback for MST

Reduce the link parameters after a link training failure for MST
outputs, similarly to how this is done for SST.

For now allow the reduction only by staying in the 8b/10b vs. 128b/132b
mode. Enabling the mode switch is left for a follow-up patchset, after
taking measures ensuring that the mode switch happens properly. In
particular a rediscovery of the whole MST topology may be required for
such a switch, see the References below.

Link: https://gitlab.freedesktop.org/drm/i915/kernel/-/issues/10970Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240610164933.2947366-17-imre.deak@intel.com

drivers/gpu/drm/i915/display/intel_dp.c

@@ -5887,11 +5887,7 @@ intel_dp_detect(struct drm_connector *connector,
 
 	intel_dp_mst_configure(intel_dp);
 
-	/*
-	 * TODO: Reset link params when switching to MST mode, until MST
-	 * supports link training fallback params.
-	 */
-	if (intel_dp->reset_link_params || intel_dp->is_mst) {
+	if (intel_dp->reset_link_params) {
 		intel_dp_reset_link_params(intel_dp);
 		intel_dp->reset_link_params = false;
 	}

drivers/gpu/drm/i915/display/intel_dp_link_training.c

@@ -1124,6 +1124,10 @@ static int reduce_link_rate(struct intel_dp *intel_dp, int current_rate)
 
 	new_rate = intel_dp_common_rate(intel_dp, rate_index - 1);
 
+	/* TODO: Make switching from UHBR to non-UHBR rates work. */
+	if (drm_dp_is_uhbr_rate(current_rate) != drm_dp_is_uhbr_rate(new_rate))
+		return -1;
+
 	return new_rate;
 }
 
@@ -1141,15 +1145,6 @@ static int intel_dp_get_link_train_fallback_values(struct intel_dp *intel_dp,
 	int new_link_rate;
 	int new_lane_count;
 
-	/*
-	 * TODO: Enable fallback on MST links once MST link compute can handle
-	 * the fallback params.
-	 */
-	if (intel_dp->is_mst) {
-		lt_err(intel_dp, DP_PHY_DPRX, "Link Training Unsuccessful\n");
-		return -1;
-	}
-
 	if (intel_dp_is_edp(intel_dp) && !intel_dp->use_max_params) {
 		lt_dbg(intel_dp, DP_PHY_DPRX,
 		       "Retrying Link training for eDP with max parameters\n");